The present invention relates generally to electrode-type glow discharge devices used in the field of thin film deposition, and more particularly to a method and apparatus for applying coating by magnetron sputtering in which surface cleaning and material deposition are provided in a continuous process.
Magnetron sputtering is a well-known technique for depositing thin coatings onto objects. Sputtering of the coating onto an object occurs by generating plasma over the surface of an emitter material in a low-pressure gas atmosphere. An electrical field accelerates ions from the plasma to bombard and eject atoms from the surface of the emitter. Once ejected, the atoms travel through the gas environment and impact the surface of a target object to be coated, bonding to the target object and forming a coating layer. During the deposition process, a high ratio of ion-to-neutral fluxes is desirable to produce a dense, hard film with a low stress.
Prior to bombarding the target to form the coating layer, however, the surface of the target object must be clean and free from impurities. Generally, surface cleaning is performed through the process of sputter cleaning, in which the surface of a target object is bombarded with ions generated by a magnetron. A high current density bombardment of the surface of the target object is desirable in order to ensure a clean surface, and along with the use of a high ratio of ion-to-neutral fluxes during the deposition process, will produce a high quality film.
Presently, there are two methods used to accomplish the surface cleaning. In the first, a separate system for the sputter-cleaning process may be used, where the plasma is generated proximate the surface of the target object, and then the surface is bombarded with ions from the plasma that are drawn to the substrate by a filament or an RF source. After the sputter-cleaning process is completed, the target object can be transferred into the coating system for film deposition. This method is inconvenient and does not guarantee a clean surface, since oxidation will occur during the interruption between sputter-cleaning and sputter-coating. In the second method, the same system may be used for sputter-cleaning and the film deposition process. Theoretically, a plasma can be generated by turning on the magnetron and applying a voltage to the target object. Ions can then be drawn to the surface of the target object with a bias voltage. If the bias voltage is sufficiently high, the ions will cause sputtering of the surface. When the bias voltage is reduced, the film deposition process may begin. This is a typical approach used in most magnetron sputtering systems. Although this approach appears, on the surface, to be simple and without interruption during the transition, it has significant shortcomings. In order to increase the sputter-cleaning rate to remove native oxides and prevent the surface from re-oxidation, the bias voltage and/or the current density must be increased. In addition, during the cleaning, film deposition should be avoided. However, with existing magnetron systems wherein the plasma is generated by the magnetron, during the cleaning process the filament material is also sputtered and unavoidably deposited onto the target object. Moreover, an increase in the magnetron power, which results in an increased current density, will not help because it also increases the emitter material ionization, resulting in an increased rate of film deposition on the surface of the target object. Although an increase in the substrate voltage would increase the bombardment rate, the magnetron must be operated to provide ions for bombardment, which necessarily would result in film deposition. This hinders the cleaning of the surface oxides, which exist on almost all metals. Also, with this method, when the film deposition process begins, to obtain a high ion flux, the magnetron power has to be increased. At the same time, the flux of neutrals sputtered from the emitter material becomes proportionally higher. As a result, the ion-to-neutral ratio remains nearly constant and endangers the quality of the film. Therefore, a compromise must be made in which a low current density with a high bias voltage must be used in order to minimize film deposition.
In addition to the problems associated with current attempts to combine the sputter-cleaning and deposition processes, in many sputter coating apparatuses, the sputtering voltage is applied with respect to end plates residing substantially perpendicular to the surface to be coated. Because the strength of the magnetic field varies along the distance between the end plates (e.g. along the surface of the target object), the non-uniformity of the magnetic field can result in a non-uniform coating. This is particularly true in the case where the target object is elongated, requiring an increased distance between the end plates.
Therefore, a need exists in the art to provide an integral sputter-cleaning and film deposition mechanism wherein the current density can be as high as necessary to effectively support the sputter-cleaning process without causing the ionization of the emitter material that results in film deposition. A further need exists to provide a mechanism for generating a uniform electric field with respect to the surface of the target object to be coated such that the cleaning and deposition is uniform along the surface of the object coated.
The following references are provided for further reference regarding magnetron sputter deposition:
M. Minato and Y. Itoh, xe2x80x9cVacuum Characteristics of TiN Film Coated on the Surface of a Vacuum Duct,xe2x80x9d Nucl. Instr. and Meth. In Phys. Res., Vol. B 121, 1997, pp. 187-190.
S. Penfold and J. A. Thornton, U.S. Pat. No. 4,030,996, Jun. 21, 1997.
N. Hosokawa, T. Tsukada and T. Misumi, xe2x80x9cSelf-Sputtering Phenomena in High-Rate Coaxial Cylindrical Magnetron Sputtering,xe2x80x9d J. Vac. Sci. Technol. Vol. 14, No. 1, 1977, pp. 143-146.
R. Wei, xe2x80x9cLow-Energy, High-Current-Density Ion Implantation of Materials at Elevated Temperatures for Tribological Applications,xe2x80x9d Surf. Coat. Technol., Vol. 83, 1996, pp. 218-227.
J. N. Matossian, R. Wei, J. Vajo, G. Hunt, M. Gardos, G. Chambers, L. Soucy, D. Oliver, L. Jay, C. M. Tylor, G. Alderson, R. Komanduri and A. Perry, xe2x80x9cPlasma-Enhanced, Magnetron-Sputtered Deposition (PMD) of Materials,xe2x80x9d Surf. Coat. Technol., Vol. 108-109, 1998, pp. 496-506.
The present invention is directed toward a new and different plasma enhanced coaxial magnetron sputtering system that is suitable for depositing a thin film of sputtered material onto a substrate. Unlike the prior art devices, the present invention utilizes two steps for depositing a thin film, instead of one.
The plasma enhanced coaxial magnetron (PECM) assembly consists of a cooling system, ring magnets, a cylindrical sputtering target material, electron emitter filaments, a cylindrical meshed anode, and power supplies. This assembly is placed in the center of a cylindrical substrate that is to be coated. Both the PECM assembly and the tube substrate are housed in a vacuum chamber. The operation of this apparatus is detailed in two steps: sputter cleaning without target material deposition and uniform sputter coatings of cylindrical substrate.
When the vacuum system is pumped down, typically to the low 10xe2x88x926 Torr range, a working gas (Ar) is introduced to the chamber to a pressure typically of a few milli-Torr. Then, an AC voltage Vf is applied to the filaments to heat them up to a thermionic temperature (xcx9c2000xc2x0 C. for tungsten). Electrons are then generated. With the application of a DC voltage Vd between the anode and the filaments, the electrons will migrate to the anode. Due to the strong magnetic field generated by the ring magnets, the electrons will experience many collisions with the gas before reaching the anode, resulting in high ionization of the gas, thereby producing an intense plasma. A negative voltage Vb is then applied to the substrate, resulting in a removal of oxides from the substrate and effective sputter cleaning.
It has been observed that two types of oxide may occur in plasma processing of materials. One is the xe2x80x9cnative oxidexe2x80x9d that forms naturally on many materials when they are exposed to ambient environments. The other is a re-oxidation that forms on the substrate surface during plasma processing due to the outgassing of water moisture adsorbed on the substrate and the vacuum chamber. Without additional care being taken, the thickness of the oxide due to re-oxidation could exceed the native oxide removed in the previous step.
Depending on the vacuum system and the substrate utilized, outgassing may take 30 minutes to several hours. In ideal conditions, during this time period, no film depositation should occur. In order to remove the native oxide and prevent the surface from re-oxidation, the sputtering rate should be maintained at greater than the sticking rate of water molecules. The sputter-cleaning rate depends on both the ion current density and the ion energy at the substrate.
In the present invention, the high ion current density comes from the discharge power of the filament current and the discharge voltage Vd and is enhanced by the magnetic field. The current density is much higher than that produced by the magnetron alone. After the outgassing process has diminished, and the surface oxide has been removed, and with the ion bombardment continuing, the magnetron power supply VT is turned on. Now sputtering of the target material starts, as does the deposition of the film. During the transition from the sputter cleaning to the film deposition, the substrate bias Vb remains high (xcx9c100-1000V) for set time, and then reduces to a low level (xcx9c50V) to ensure a good interface. Since in this technique the current density and the ion energy can be controlled separately, the broad range of requirements for the cleaning and film deposition can be met readily.